The Fischer-Tropsch process is often used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas, coal-bed methane, residual oil stream, biomass, and/or coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas). The synthesis gas is then converted in a second step over a suitable catalyst at elevated temperature and pressure into paraffinic and olefinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
The obtained product (Heavy Paraffin Synthesis (HPS) product) may be fed to a hydroisomerisation/hydrocracking unit, but is preferably first fed to a hydrogenation unit where the olefins and oxygenates are hydrogenated. In the hydrogenation unit there is no or substantially no hydroisomerisation and/or hydrocracking. Some hydrogenated product may be removed at this point for sale but most of the hydrogenated products proceed to a hydroconversion unit, especially the C5+ fraction, in which hydroisomerisation as well as hydrocracking occurs.
The presence of certain non-hydrogenated oxygenates in the otherwise hydrogenated products can reversibly deactivate the catalyst used in the hydrogenisation and/or the hydroisomerisation/hydrocracking unit (or heavy paraffin conversion (HPC) unit). In particular, complex oxygenates produced by an aldol type reaction, or acetals, are thought to be particularly responsible for the deactivation of the catalyst used in these reactions. In addition, it is sometimes desired to produce products which are completely free from oxygen compounds. This holds, for instance, for wax products, i.e. C30+ wax products, especially C40+ wax products.
An object of the present invention is to mitigate the deactivation of the HPC catalyst, and especially the hydrogenation catalyst.